atusb/fw2/: move board-specific items to board-specific file

- io.h: renamed to board.h
- Makefile (OBJS): added board.o
- atusb.c (reset_rf): moved to board.c
- board.h (reset_rf): give reset_rf a proper declaration
- atusb.c, ep.c, spi.c: include board.h instead of io.h

atusb/fw2/board.c

@@ -0,0 +1,20 @@
+#include <avr/io.h>
+
+#define F_CPU   8000000UL
+#include <util/delay.h>
+
+#include "board.h"
+
+
+void reset_rf(void)
+{
+	/* AT86RF231 data sheet, 12.4.13, reset pulse width: 625 ns (min) */
+
+	CLR(nRST_RF);
+	_delay_us(1);
+	SET(nRST_RF);
+
+	/* 12.4.14: SPI access latency after reset: 625 ns (min) */
+
+	_delay_us(1);
+}